Examples of a method of performing densification to an optical disc, include a method of performing densification in a linear density direction by shortening a channel bit length, namely, a mark length, and a method of narrowing a track pitch. However, performing the densification in the linear density direction causes a problem that inter-code interference increases. In addition, narrowing the track pitch increases leakage of information from an adjacent track (adjacent track crosstalk). A method of reducing adjacent track crosstalk (hereinafter, appropriately referred to as crosstalk, simply) has been proposed.
For example, Patent Document 1 describes that crosstalk is canceled by supplying reproduced signals of a track to be reproduced and tracks on both sides thereof, to adaptive equalizer units, and controlling the tap coefficients of the adaptive equalizer units.
Furthermore, Patent Documents 2 and 3 each describe that the influence of crosstalk is reduced by spatially dividing reflected light from an optical medium into three in a track width direction, individually detecting the light that has been divided into three, performing constant multiplication to detected signals (weighting), and operating addition. In addition, Patent Document 2 has suggested that further performing weighting in a beam propagating direction can emphasize and reproduce a reproduced signal of a small recorded mark, as an idea.